1. Field of the Invention
The present invention relates to an eye examining instrument, and more particularly to a system for focusing the instrument to the fundus of the eye to be examined. The invention further relates to an improvement over U.S. patent application Ser. No. 109,275, now abandoned in favor of continuation U.S. patent application Ser. No. 346,870 by one of the inventors.
2. Description of the Prior Art
In a conventional eye-fundus camera, the focusing to the eye fundus has been achieved by adapting the sight of the eye to cross-hairs in the finder and by adjusting the photographing lens in such a manner that the eye fundus can be clearly viewed in this state. However, such focusing method inevitably involves personal fluctuation and is difficult to conduct rapidly.
U.S. Pat. No. 3,016,000 discloses a method of focusing the camera to the eye fundus by projecting a focus mark onto the eye fundus and adjusting the focusing lens so that the focus mark can be clearly viewed. Also, U.S. Pat. No. 3,925,793 (German Pat. No. 2,415,319) and U.S. patent application Ser. No. 945,845 disclose a focusing method by projecting plural marks on the eye fundus and aligning said marks under observation. Although a rapid focusing has been rendered possible by these methods, there still remains a strong demand for automatic focusing of the camera, as the setting of an eye-fundus camera simultaneously requires the alignment of the eye axis with the optical axis of the objective lens, the distance adjustment between the cornea and the objective lens and the focus adjustment. The operator has to constantly pay attention to these three factors as the above-mentioned alignment and distance are easily affected by small movements of the subject to be examined while the focusing is affected by a change in the sight of the eye to be examined. For this reason automatic focusing, if realized, will significantly alleviate the load on the operator and contribute to the probability of obtaining photographs of improved image quality. Also, in case of continuous photographing with successively displaced viewing fields, it becomes necessary to correct the focusing for each field displacement, and automatic focusing will contribute to the image quality in such case.
A pioneer invention for automatic focusing of the eye-fundus camera to the eye fundus is shown in U.S. Pat. No. 3,614,214, in which a dichroic mirror reflecting the infrared light but transmitting the visible light is provided in front of an ordinary eye-fundus camera in an oblique position to deflect the detecting beam from an automatic optometer toward the eye to be examined and to again deflect the reflected beam from the eye to said optometer, whereby the focusing lens of the eye-fundus camera is adjusted by the output of said optometer.
Now, even if the focusing to the eye fundus has come to be automatically executed, some time is required from the start of the detection until the adjustment is completed, and the examinee often moves and the refractive power of the eye fluctuates and therefore, the focus detecting apparatus always operates following the movement of the examinee and the variation in refractive power with a minute time delay. If the timing of the photographing release is bad, there will occur a disadvantage that photography is effected at a point of time whereat the focusing is still insufficient and thus, it is desirable that the completion of the focusing can be visually confirmed.
Also, where the crystalline lens of the examinee is turbid, it is often the case that the focus detecting apparatus does not sufficiently function and in such case, it is preferable that manual focus adjustment can be effected and further, since the retina is more or less thick, the necessity of manual focus adjustment occurs when it is desired to shift the focus of the camera to another position on the retina. In these cases, it is convenient to effect rough focusing to the eye fundus and then effect delicate focusing and for this purpose, a focus mark is caused to appear in the finder view field so that the degree of the focusing can be visually confirmed.
On the other hand, the aforementioned U.S. patent application Ser. No. 109,275, abandoned in favor of continuation U.S. patent application Ser. No. 346,870 has proposed a method whereby an index mark is projected upon the eye to be examined and the image thereof reflected by the eye fundus is directed onto two adjacent photoelectric conversion elements so that the focus adjusted condition can be known from the magnitude of the difference between the outputs of the two elements. In this method, where non-uniform background noise has ridden on the index mark, for example, where the brightness of the eye fundus around the index mark is non-uniform or where strong noise has ridden on the circumference of the index mark due to the reflection of the cornea, a great differential output and accordingly an error occurs in spite of the index mark lying at a predetermined position. Also, the light intensity of the index mark varies in accordance with movement of the eyeball and the differential output varies correspondingly, and this is inconvenient to the focus adjustment. Further, it has been empirically found that when the index mark is greatly deviated from the in-focus position, there is no difference between the two outputs and this is sometimes judged as in-focus by mistake.